1. Field of the Invention
The invention relates to a tunable laser source of a Littman arrangement type, including a semiconductor laser and a diffraction grating which are fixed to an optical base, and a wavelength selection mirror which rotates around a position, as a rotation center, where a mode hop is suppressed at a time of a wavelength variation. The invention particularly relates to a tunable laser source in which a wavelength error with respect to influence of a temperature change can be suppressed without using a heat insulation structure.
2. Description of the Related Art
FIG. 5 shows the configuration of an external resonator type tunable laser source as a related art (For example, see Document 1 and Document 2.). With reference to FIG. 5, an external resonator type tunable laser source in a Littman arrangement will be described as an example. An optical amplifier 10 has a semiconductor laser 11, a first lens 12, and a second lens 13. The semiconductor laser 11 has an antireflection film 11a at one end. A wavelength selection section 20 has a diffraction grating 21 and a wavelength selection mirror 22, selects the wavelength of the light to be emitted from the one end of the optical amplifier 10, and feedbacks the selected light to the optical amplifier 10.
Light emitted from the one end (the end face where the antireflection film 11a is formed) of the semiconductor laser 11 is converted to parallel light by the first lens 12, and then enters the diffraction grating 21. The light entering the diffraction grating 21 is diffracted by the diffraction grating 21, wavelength-dispersed to different angles depending on the wavelength, and then enters the wavelength selection mirror 22. Among the light incident on the wavelength selection mirror 22, only the light of a desired wavelength is reflected to the diffraction grating 21 through the same optical path.
The light incident on the diffraction grating 21 is again wavelength-dispersed. Only the light of the wavelength selected by the wavelength selection section 20 is converged in the semiconductor laser 11 by the first lens 12 to be fed back. The other end of the semiconductor laser 11, and the wavelength selection mirror 22 form an external resonator, and perform laser oscillation.
By contrast, the laser light emitted from the other end which is not provided with the antireflection film 11a is converted to parallel light by the second lens 13. The parallel light is passed through an isolator which is not shown, and emitted as output light.
The rotation center P1 of the wavelength selection mirror 22 is a point at the intersection of an extension of the diffracting plane of the diffraction grating 21, that of the reflecting plane of the wavelength selection mirror 22, and that of a plane constituting the external resonator (strictly speaking, slightly shifted by the refractive index of the semiconductor laser 11 to the side of the second lens 13 with respect to the reflecting face or the other end of the semiconductor laser 11). The arrangement of the semiconductor laser 11, the diffraction grating 21, and the wavelength selection mirror 22 is called a Littman arrangement (or a sine-bar arrangement). In the Littman arrangement, generation of a mode hop can be prevented from occurring over a broad band, and a continuous wavelength sweep with a reduced power variation is enabled. Namely, the rotation center P1 is located at a position where a mode hop is suppressed when a wavelength is tuned.
A rotation shaft of a mirror arm 31 is disposed at the rotation center P1, and the wavelength selection mirror 22 is fixed to the mirror arm. A stepping motor (hereinafter, abbreviated to motor) 32 is contacted with one end of the mirror arm 31 via a screw 33. A tension spring 34 is disposed in parallel with the screw 33.
For example, the motor 32 is rotated to cause the screw 33 to push the mirror arm 31 (in FIG. 5, in the leftward direction). This causes the mirror arm 31 to be rotated about the rotation center P1 while maintaining the Littman arrangement. Therefore, the wavelength selected by the wavelength selection section 20, and the optical path length of the diffraction grating 21 and the wavelength selection mirror 22 are changed, and also the wavelength of the output light is changed toward a shorter wavelength side.
When the motor 32 is rotated in the opposite direction, the screw 33 is moved in the direction along which the screw is separated from the mirror arm 31 (in FIG. 5, in the rightward direction), and the mirror arm 31 is pulled toward the motor 32 by the tension spring 34. This causes the mirror arm 31 to be rotated about the rotation center P1 while maintaining the Littman arrangement. Therefore, the wavelength selected by the wavelength selection section 20, and the optical path length of the diffraction grating 21 and the wavelength selection mirror 22 are changed, and also the wavelength of the output light is changed toward a longer wavelength side.
The mirror arm 31 to which the wavelength selection mirror 22 is fixed is rotated about the rotation center P1, whereby a wavelength sweep is performed.
The following documents are referred to as related art.
[Document 1] G. Littman et al., “Novel Geometry for single-mode scanning of tunable lasers,” OPTICAL LETTERS, Optical Society of America, March 1981, Vol. 6, No. 3, pp. 117–118.
[Document 2] JP-A-2002-190642 (paragraph [0002] to [0004], FIGS. 6 to 8).
In the tunable laser source shown in FIG. 5, the wavelength is tuned by rotating the wavelength selection mirror 22. In order to stably output a desired wavelength, the length of the external resonator extending from the semiconductor laser 11 to the wavelength selection mirror 22 via the diffraction grating 21 must be always constant. In order to make the tunable laser source free from a mode hop, a sine-bar arrangement must be employed as shown in FIG. 5.
Usually, the optical amplifier 10, the wavelength selection section 20, and the mirror arm 31 are arranged on an optical base which is not shown. Therefore, thermal expansion or contraction is caused in the optical base itself by a change of the ambient temperature or a variation of heat generation of the semiconductor laser 11. Consequently, the positional relationships of the optical amplifier 10, the wavelength selection section 20, and the mirror arm 31 which are arranged on the optical base are varied, and the length of the external resonator is changed, thereby producing a problem in that a wavelength error occurs.
In order to eliminate the influence of thermal expansion or contraction of an optical base, it is matter of course that the whole of a tunable laser source may be configured as a heat insulation structure. However, the size of the entire light source is very large.